To provide a building material which less easily generates any whitening phenomenon or cracking in a folded part, or generates any excessive phenomenon of a sheet in the folding part, and provide a V-groove cutting edge suitable for the manufacture of the building material.

In the building material in which has the sheet 1 stuck to a face side of a flat core 2, forms a V-groove 4 on the back side of the core 2, and is folded to the back side of the core 2 by making use of the V-groove 4 to form the folded part, a flat portion 6 of the width of 0.4-0.8 mm is formed at a tip of the V-groove 4, and a remaining margin 7 of 0.1-0.3 mm of the core is formed at the position of the tip of the V-groove. A flat portion of the width of 0.4-0.8 mm is formed at a tip of the V-groove cutting edge.